Four proteins (UreD, UreE, UreF, and UreG) are involved in the process of selectively incorporating nickel ion into urease apoprotein. The functions of each of these proteins are being explored by a methods that include efforts to overproduce and purify each protein, examination of deletion or specific site-directed mutants of these auxiliary factors, and testing for functional activities. One hypothesis being tested is that one or more of these proteins may catalyze or undergo selective chemical modification, as part of the activation process. We are using mass spectrometric methods to examine the mass of the individual purified proteins from wild-type and mutant cells.